1. Field of Invention
The invention relates to methods and systems that compensate for noise in digitized speech.
2. Description of Related Art
As telecommunications plays an increasingly important role in modern life, the need to provide clear and intelligible voice channels increases commensurately. However, providing clear, noise-free and intelligible voice channels has traditionally required high-bit-rate communication links, which can be expensive. While lowering the bit-rate of a voice channel can reduce costs, low-bit-rates tend to introduce side-effects, such as quantization noise, which can reduce the clarity and/or intelligibility of voice signals. Unfortunately, removing noise in a voice signal generated by low-bit-rate channels can require excessive processing power and distort the voice signal. Accordingly, there is a need for new technology to provide better voice channels that reduce processing power requirements while minimizing distortion.
The invention provides the short-term post-filtering methods and systems for digital voice communications. Generally, post-filtering improves the perceptual quality of the synthesized signal and is widely used in current low-bit-rate speech coders. The common post-filter consists of three filters: a long-term post-filter, a short-term post-filter and a tilt compensation filter. The long-term post filter generally relates to improving perceptual quality of speech by emphasizing pitch periodicity. The short-term post filter, adaptively constructed from LPC coefficients, removes perceptible noise from synthesized or reconstructed speech by de-emphasizing speech frequency components related to spectral valleys, or local minima. The tilt compensation filter is required to compensate for spectral tilt caused by the short-term post-filter.
In various exemplary embodiments, a set of linear predictive coding (LPC) coefficients is used to derive a second set of LPC coefficients having a reduced order, which can subsequently be used to derive a low-order short-term post-filter based on the pseudo-cepstrum. The low-order short-term post-filter can then adaptively remove perceptible noise from synthesized or reconstructed speech by emphasizing speech frequency components related to the formants of the LPC coefficients and de-emphasizing speech frequency components related to the spectral valleys of the LPC coefficients. The short-term post-filter can also compensate for spectral distortion such as spectral tilt and minimize phase distortion.
Other features and advantages of the present invention will be described below or will become apparent from the accompanying drawings and from the detailed description which follows.